Fuel cells are receiving increasing attention as a viable alternative energy system. In general, fuel cells convert chemical energy into electrical energy in an environmentally clean and efficient manner, typically via oxidation of hydrogen or an organic fuel in the anodic half-cell coupled to an oxygen reduction reaction (ORR) in the cathodic half-cell. Fuel cells are contemplated as power sources for everything from small electronics to cars and homes. In order to meet different energy requirements, there are a number of different types of fuel cells in existence today, each with different chemistries, requirements, and uses.
Biofuel cells are fuel cells that rely on or mimic natural biological processes to produce power. Examples of biofuel cells include enzymatic fuel cells (EFCs), which use enzymes as the electrocatalysts and microbial fuel cells (MFCs), which use microorganisms for conversion of chemical energy to electricity
In a typical MFC anode, dissimilatory metal-reducing bacteria convert chemical energy to electrical energy by transferring electrons from reduced electron donors (e.g. lactate) to insoluble electron acceptors (i.e., the electrode surface). (See Logan et al., Environ. Sci Technol. 2006, 40, 5181-5192.) In nature, bacteria maximize the use of insoluble electron acceptors (usually Fe or Mn oxides) by excreting extracellular polymers that serve to bind the growing cell population into a structured biofilm. That anchored community is thus physically coupled to its electron acceptor, enhancing respiration processes. (See McLean et al., J. Microbiol. Methods 2008, 74, 47-56 and Yi et al., Biosens. Bioelectron. 2009, 24, 3498-3503.) Biofilms, however, require significant time to become established, which often leads to variable and irreproducible power density when applied to MFC design. (See e.g., Biffinger et al., Biosens. Bioelectron. 2007, 22, 1672-1679.)
Furthermore achieving efficient energy transfer from the microbial cells of a biofilm requires an electrode material that is conductive, yet biocompatible, in order to provide an interface for bacterial interactions. While numerous conductive and carbonaceous materials have been investigated to support anodic reactions in MFCs, many potential anode materials are restricted in application by limitations in scalability, cost-effectiveness, conforming dimensions, and manufacturability. Furthermore, many traditional methods for forming catalytic materials employ harsh chemical and physical conditions (such as acid etchants and heat treatments) that are inhospitable to biologicals.
Moreover, because MFCs (and EFCs) rely on the activity of live cells (or other biologically active materials), biologically-compatible methods for forming electrodes incorporating biologically active components are needed.